Communication apparatus and communication method

BACKGROUND OF THE INVENTION

00011. Field of the Invention

0002The present invention generally relates to power line communication (PLC) systems and methods for data communications over a power distribution system, and more particularly, to a device for facilitating communications through power lines and a method of using the same.

00032. Related Art

0004In general, when a terminal (for example, a computer terminal) performs wired data communication in a home, office or factory environment, a great deal of preparatory engineering work must be performed before a communication system including the terminal is able to begin operations. This is because the communication system needs to install connectors and cables as transmission lines to appropriate positions.

0005However, with respect to power line communications, the preparatory engineering work in such environments is reduced in comparison with other communication technologies, because almost all of such environments already have in place many commercial power supply lines located in virtually every nook and cranny, while using a commercial power supply, for example, alternating current 100V (50 Hz/60 Hz) in Japan or 120V in the U.S. More particularly, in the case of PLC, it will be possible to establish a data communication line by just connecting a communication apparatus plug to an outlet of the commercial power supply.

0006JP2000-165304A describes an example of PLC technology utilizing a power line as a data communication line.

0007In Japan, the frequency band from 2 MHz to 30 MHz is planned to be opened to PLC. At present, many companies are in the process of research and development of PLC technology. However, at this time, there is no PLC standard in Japan, and each company has different specifications for PLC communications relating to protocol, modulating method, and frequency band.

0008In view of the above, there is high possibility, during actual use, of mixing different PLC communication methods in a same environment. For example, assuming that people who live in an apartment or condominium complex use PLC apparatuses therein, they may use different communication apparatuses made by different manufacturers. In such situation, these different communication apparatuses may be simultaneously connected to a common power line.

0009In such situation, each communication apparatus may not demodulate signals from the other apparatuses that use different types of PLC communication methods, and may recognize these signals as noise. Because each apparatus may not recognize the existence of the others on the common power line, signals output from different communication apparatuses may collide with each other. Under these conditions, it may become almost impossible to effect a communication. That is, it may become almost impossible for these different PLC apparatuses to coexist on a common power line. In order to facilitate such coexistence, these different apparatuses would need to undergo significant changes to their circuits and controllers.

0010On the other hand, a plurality of the same kind of PLC apparatuses are able to communicate with each other using a common power line because the signal multiplexing is performed based on time division multiplexing.

SUMMARY

0011Some embodiment examples described herein address the above-mentioned problem.

0012According to an embodiment example, the communication apparatus according to the invention comprises a synchronizing signal generator that generates a synchronizing signal based on a timing of an alternating waveform in a power line, a data communicating circuit that performs the data communication, and a communication controller that controls to acquire a transmitting right utilizing a timing of the synchronizing signal and to control the communication circuit in accordance with whether or not the communication apparatus acquires the transmitting right.

BRIEF DESCRIPTION OF THE DRAWING

0013<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram illustrating a communication apparatus;

0014<figref idref="DRAWINGS">FIG. 2</figref> is a timing chart illustrating an example of an operation of a plurality of communication apparatuses;

0015<figref idref="DRAWINGS">FIG. 3</figref> is a graph illustrating an example of a frequency characteristic of a transmission line using a power line;

0016<figref idref="DRAWINGS">FIG. 4</figref> is a graph illustrating an example of a noise frequency characteristic of a wall outlet;

0017<figref idref="DRAWINGS">FIG. 5</figref> is a block diagram illustrating an example of a communication controller;

0018<figref idref="DRAWINGS">FIG. 6</figref> is a wave-shape diagram illustrating an example of a signal format outputted from a communication controller;

0019<figref idref="DRAWINGS">FIG. 7</figref> is a diagram illustrating an example of a signal spectrum of control signals outputted from a communication controller;

0020<figref idref="DRAWINGS">FIG. 8</figref> is a timing chart illustrating an example of an operation of a plurality of communication apparatuses;

0021<figref idref="DRAWINGS">FIG. 9</figref> is a timing chart illustrating an example of an operation of a plurality of communication apparatuses;

0022<figref idref="DRAWINGS">FIG. 10</figref> is a block diagram illustrating an example of a system connected a plurality of communication apparatuses to a common transmission line in an apartment complex;

0023<figref idref="DRAWINGS">FIG. 11</figref> is a flow chart illustrating an example of an operation for resolving a hidden apparatus problem;

0024<figref idref="DRAWINGS">FIG. 12</figref> is a timing chart illustrating an example of an operation of a plurality of communication apparatuses;

0025<figref idref="DRAWINGS">FIG. 13</figref> is a timing chart illustrating an example of an operation of a plurality of communication apparatuses;

0026<figref idref="DRAWINGS">FIG. 14</figref> is a timing chart illustrating an example of an operation of a plurality of communication apparatuses;

0027<figref idref="DRAWINGS">FIG. 15</figref> is a timing chart illustrating an example of an operation of a plurality of communication apparatuses;

0028<figref idref="DRAWINGS">FIG. 16</figref> is a block diagram illustrating an example of a system connected a plurality of communication apparatuses to a common transmission line;

0029<figref idref="DRAWINGS">FIG. 17</figref> is a circuit block diagram illustrating a communication apparatus shown in <figref idref="DRAWINGS">FIG. 1</figref>.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENT EXAMPLES

0030Several embodiment examples will be described below with reference to <figref idref="DRAWINGS">FIGS. 1 and 17</figref>.

0031As shown in <figref idref="DRAWINGS">FIG. 1</figref>, a communication apparatus <b>100</b> is electrically connected to a transmission line <b>200</b>. In this embodiment example, the communication apparatus operates as a modem, and communicates with a communication terminal (not shown) such as a computer. In particular, the communication apparatus <b>100</b> may be a personal computer, a home information appliance, an Internet appliance, or a digital network appliance, or other such devices.

0032A power line installed in a home, an office or a factory, for example, rubber-insulated cable, is used as transmission line <b>200</b>. The power line supplies electricity of a commercial power supply, for example, Alternating Current 100V (50 Hz/60 Hz) in Japan, to each electric apparatuses. The power line may be able to utilize unused frequency band for data communication.

0033In addition, the commercial power supply is not required to be alternating current 100V (50 Hz/60 Hz) but each country has an original standard such as alternating current 120V (60 Hz) in the U.S. and alternating current 110/220V (50 Hz) in China.

0034Furthermore, when a communication apparatus <b>100</b> is installed in an apartment or a condominium complex, as shown in <figref idref="DRAWINGS">FIG. 16</figref>, many communication apparatuses <b>100</b> connect to the transmission line <b>200</b>. Referring to <figref idref="DRAWINGS">FIG. 16</figref>, for example, a plurality of the communication apparatuses <b>100</b>A<b>1</b>, <b>100</b>A<b>2</b>, <b>100</b>B<b>1</b>, <b>100</b>B<b>2</b>, <b>100</b>C<b>1</b>, and <b>100</b>C<b>2</b> connect to the power line <b>200</b>. A pair of the communication apparatuses <b>100</b>A<b>1</b> and <b>100</b>A<b>2</b>, a pair of the communication apparatuses <b>100</b>B<b>1</b> and <b>100</b>B<b>2</b>, and a pair of the communication apparatuses <b>100</b>C<b>1</b> and <b>100</b>C<b>2</b> use communication method “A”, communication method “B”, and communication method “C”, respectively. Therefore, each or the communication apparatuses <b>100</b>A<b>1</b> and <b>100</b>A<b>2</b> is the same type of the communication apparatus <b>100</b>A. Each of the communication apparatuses <b>100</b>B<b>1</b> and <b>100</b>B<b>2</b> is the same type of the apparatus <b>100</b>B. Each of the communication apparatuses <b>100</b>C<b>1</b> and <b>100</b>C<b>2</b> is the same type of the apparatus <b>100</b>C.

0035However, the communication types A, B and C among the communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C are different from each other. The difference/differences among those three types of communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C is/are at least one of communication protocol, modulation method of data signal, symbol rate of the data signals, and so on.

0036Referring to <figref idref="DRAWINGS">FIGS. 1 and 17</figref>, a technology will be described, which allows a plurality types of communication apparatuses to coexist on a common transmission line <b>200</b>. Hereinafter, it is assumed that there are three communication apparatus <b>100</b>A, <b>100</b>B and <b>100</b>C on the transmission line <b>200</b>. Each of the communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C may have a data communication circuit <b>110</b>, a communication controller <b>120</b>, an AC cycle detector <b>130</b>, and a switching circuit <b>140</b> in common.

0037For illustrative purposes, hereinafter, a structure of the communication apparatus <b>100</b>A will be described in detail.

0038Communication apparatus <b>100</b>A further comprises a circuit module <b>150</b> and a switching power source <b>160</b>. The switching power source <b>160</b> supplies several kinds of power voltages, for example +1.2V, +3.3V and +12V, to the circuit module <b>150</b>. The circuit module <b>150</b> comprises a coupler <b>170</b>, a band pass filter <b>171</b>, an ADC IC <b>176</b>, a memory <b>177</b> and an Ethernet PHY (Physical layer) IC <b>174</b> in addition to the data communication circuit <b>110</b>, the communication controller <b>120</b>, the AC cycle detector <b>130</b> and the switching circuit <b>140</b>

0039The data communication circuit <b>110</b> is an electric/electronic circuit that performs signal processing including general control and modulation/demodulation for the data communication as a typical modem. The data communication circuit <b>110</b> modulates a data signal or data signals (hereinafter data signals) outputted from a terminal such as a personal computer (not shown) to provide modulated signals, and outputs the modulated signals as transmitted data signals. Furthermore, the data communication circuit <b>110</b> demodulates data signals inputted through the transmission line <b>200</b> to provide demodulated signals, outputs the demodulated signals as received data signals to a communication terminal such as a personal computer. In addition, the data communication circuit <b>110</b> outputs predetermined communication request signals in advance of the data communication in order to confirm the condition of the transmission line <b>200</b> including whether or not the transmission line <b>200</b> is ready for the data communication.

0040The data communication circuit <b>110</b> comprises a main IC which comprises a CPU (central processing unit) <b>111</b>, a PLC/MAC block (power line communication/media access control layer block) <b>112</b> and a PLC/PHY block (power line communication/physical layer block) <b>113</b>. The CPU <b>111</b> comprises a 32 bits RISC (reduced instruction set computer) processor. The PLC/MAC block <b>112</b> manages MAC layer of received and transmitted signals. The PLC/PHY block <b>113</b> manages PHY layer of the received and transmitted signals.

0041The switching circuit <b>140</b> is located between the data communication circuit <b>110</b> and the transmission line <b>200</b>, and has a plurality of switches, which control to pass the transmitted data signals and the received data signals. In other words, the plurality of switches control to switch the data communication function. The switching circuit <b>140</b> comprises analog front end (AFE) IC <b>141</b>, a low pass filter <b>142</b>, a band pass filter <b>143</b> and a driver IC <b>144</b>. The AFE IC <b>141</b> comprises many devices such as an analog/digital (A/D) converter <b>141</b><i>a</i>, a D/A converter <b>141</b><i>b</i>, filters and a VGA (variable gain amplifier) <b>141</b><i>c</i>, and is an interface between the data communication circuit <b>110</b> and the transmission line <b>200</b>. The switching circuit <b>140</b> controls to pass the transmitted data signals and/or the received data signals by switching these elements in the AFE IC <b>141</b>.

0042Furthermore, the switching circuit <b>140</b> may comprise a switch, which can be controlled to switch by an external control signal, like an analog switch. In that case, the AFE mentioned before may be incorporated in the data communication circuit <b>110</b>. It will be recognized by those skilled in the art that many alternative kinds of switches can be used as the switching circuit <b>140</b> if the switches can switch the data communication function.

0043The AC cycle detector <b>130</b> produces synchronizing signals, which is used such that a plurality types of the communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C control in a common timing. The AC cycle detector <b>130</b> comprises a diode bridge <b>131</b>, registers <b>132</b> and <b>133</b>, a DC (direct-current) power supply <b>134</b> and a comparator <b>135</b>. The diode bridge <b>131</b> connects to the register <b>132</b>. The register <b>132</b> connects to the register <b>133</b> in series. Both the registers <b>132</b> and <b>133</b> connect to a terminal of the comparator <b>135</b> in parallel. The DC power supply <b>134</b> connects to another terminal of the comparator <b>135</b>. Practically, as shown in <figref idref="DRAWINGS">FIG. 1</figref>, the AC cycle detector <b>130</b> detects points at the intersection of X-axis with the 50 Hz or 60 Hz AC voltage sine-waveform (AC) of the commercial power source supplied to the transmission line <b>200</b> (zero crossing points), produces synchronizing signals (SS) with reference to the zero crossing points, and outputs the SS. Each SS may be, for example, a rectangular wave including a plurality of pulses synchronizing to the zero crossing point. Therefore, the SS may start from the zero crossing point or may have a certain offset from the zero crossing point.

0044The communication controller <b>120</b> performs a control necessary to coexist with the other communication apparatuses <b>100</b>B and <b>100</b>C, synchronizing to the timing of the SS outputted from the AC cycle detector <b>130</b>. That is to say, the communication controller <b>120</b> controls to acquire a right that the communication apparatus <b>100</b> uses the transmission line <b>200</b> in accordance with communication request signals outputted from the data communication circuit <b>110</b>. Furthermore, the communication controller <b>120</b> outputs control signals as transmitted control signals to the transmission line <b>200</b> in order to negotiate with other communication apparatuses <b>100</b>B and <b>100</b>C with the right to use the transmission line <b>200</b> (hereinafter, the “transmitting right”), and receives control signals via the transmission line <b>200</b> as received control signals. In addition, the controller controls the switching circuit <b>140</b> in accordance with whether or not the communication apparatus <b>100</b>A acquires the transmitting right, in other words, whether or not it is a time period when the communication apparatus <b>100</b>A can use the frequency band of the transmission line <b>200</b>.

0045The communication controller <b>120</b> controls to switch the switching circuit <b>140</b> during the time period when the communication apparatus <b>100</b> can not use the frequency band on the transmission line <b>200</b>. During this time, therefore, the data communication circuit <b>110</b> disconnects to the transmission line <b>200</b>. This configuration makes it possible to prevent a plurality of different type signals outputted from a plurality of different types communication apparatuses from colliding with each other on the transmission line <b>200</b> because, during this time, only the communication apparatus <b>100</b>A can exclusively use the frequency band of the transmission line <b>200</b>. Therefore, each manufacturer can select an appropriate communication protocol, an appropriate modulation method, an appropriate symbol rate, and so on in accordance with its design concept without considering the collision with the other communication apparatuses connected to the transmission line. Further, its existing communication circuit can be used as the data communication circuit <b>110</b> without making a significant change. Moreover, at least two of these functions of the data communication circuit <b>110</b>, the communication controller <b>120</b>, the AC cycle detector <b>130</b>, and the switching circuit may be integrated into an integrated circuit.

0046The coupler <b>170</b> comprises a coil-type transformer <b>171</b>, and coupling condensers <b>172</b> and <b>173</b>. The power source connecter <b>180</b> connects to the AC cycle detector <b>130</b>, the coupler <b>170</b> and the switching power source <b>160</b>. The coupler <b>170</b> connects to the communication controller <b>120</b> and the switching circuit <b>140</b>. The switching circuit <b>140</b> connects to the data communication circuit <b>110</b> and RJ45 plug-in phone jack <b>190</b> via the Ethernet PHY IC <b>174</b>.

0047As shown in <figref idref="DRAWINGS">FIG. 2</figref>, in this embodiment example, the frequency band on the transmission line <b>200</b> is divided into a plurality of bands such as a commercial power source band <b>11</b>, a control signal band <b>12</b>, and data signal band <b>13</b>. For example, the frequency band assigned to the commercial power band is from 50 Hz to 2 MHz, the frequency band assigned to the control signal band is from 2 MHz to 3 MHz, and the frequency band assigned to the data signal band is from 3 MHz to 30 MHz.

0048The control signal band <b>12</b> is exclusively used for the negotiation to acquire the transmitting right. In other words, the control signals for the negotiation are transmitted and received via the control signal band <b>12</b>. The transmitted control signal and the received control signal shown in <figref idref="DRAWINGS">FIG. 1</figref> are assigned to the control signal band <b>12</b>.

0049The data signal band <b>13</b> is exclusively used for the actual data communication signals. Various data signals are transmitted and received through the data signal band <b>13</b>. The transmitted signal (data) and the received signal (data) shown in <figref idref="DRAWINGS">FIG. 1</figref> are assigned to the data signal band <b>13</b>.

0050As shown in <figref idref="DRAWINGS">FIGS. 3 and 4</figref>, signals transmitted in the frequency band from 2 MHz to 3 MHz tend to attenuate greater than signals transmitted in the other frequency band, and tend to have more noise than signals transmitted in the other frequency band. Although it is desirable to use frequency band as broad as possible in order to perform fast transmission, this frequency band (2–3 MHz) does not contribute much to the fast transmission because the signal-to-noise ratio (S/N) of this frequency band is relatively low. Therefore, assigning this frequency band to the control signal band <b>12</b> makes it possible to inhibit the transmission speed through the transmission line <b>200</b> from lowering.

0051As shown in <figref idref="DRAWINGS">FIG. 5</figref>, the communication controller <b>120</b> comprises a controller <b>121</b>, signal generator <b>122</b>, a D/A converter <b>123</b>, a low pass filter (LPF) <b>124</b>, a band pass filter (BPF) <b>125</b>, an AGC circuit <b>126</b>, an A/D converter <b>127</b>, and a fast Fourier transform (FFT) circuit <b>128</b>.

0052The controller <b>121</b> is a digital circuit, which controls the entire communication apparatus <b>100</b>A in accordance with the communication request signal, synchronizing the timing of the synchronizing signals inputted from the AC cycle detector <b>130</b>.

0053The signal generator <b>122</b> generates a waveform pattern of the control signal necessary to perform negotiation with the other communication apparatuses <b>100</b>B and/or <b>100</b>C connected to the transmission line <b>200</b> in accordance with an instruction of the controller <b>121</b>. This control signal is a multi-carrier signal such as orthogonal frequency division multiplexing (OFDM) and spread spectrum. Practically, the control signal utilizing OFDM, which has a signal spectrum as shown in <figref idref="DRAWINGS">FIG. 7</figref>, is produced as shown in <figref idref="DRAWINGS">FIG. 6</figref>.

0054The D/A converter <b>123</b> converts digital OFDM signals outputted from the signal generator <b>122</b> into analog signals.

0055The LPF <b>124</b> allow the analog signals with the frequency of the control signal band <b>12</b> to pass therethrough, and prevent the analog signal with the other frequency from passing therethrough. Again, the frequency band of the control signal band <b>12</b> is 2–3 MHz. In addition, the LPF <b>124</b> can be replaced with a band pass filter.

0056The BPF <b>125</b> allows analog signals with the frequency of the control signal band <b>12</b>, which are inputted from the transmission line <b>200</b>, to pass therethrough, and output the passed analog signals to the AGC circuit <b>126</b>.

0057The AGC circuit <b>126</b> automatically controls gain of passed analog signals, and amplifies the analog signals such that the passed analog signals from the BPF <b>125</b> can keep a specified level if the passed analog signal attenuates.

0058The A/D converter <b>127</b> converts the analog signals inputted from the AGC circuit <b>126</b> into digital signals.

0059The FFT circuit <b>128</b> performs predetermined FFT to the digital signals inputted from the A/D converter <b>127</b>, converts multi-carrier signals emerging in time domain into signals in frequency domain. Here, the FFT circuit performs FFT at 128 points. The number of the points is not limited to 128.

0060The controller <b>121</b> examines the signals outputted from the FFT circuit <b>128</b>, and confirms whether or not a carrier relevant to a signal sent by the communication apparatus <b>100</b>B or <b>100</b>C as a OFDM control signal (coexistence signal) exists in the signals outputted from the FFT circuit <b>128</b>.

0061Next, several controls performed by the communication controller <b>120</b>, which are necessary to a plurality types of the communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C so as to coexist on the common transmission line <b>200</b>, will be described.

0062As the AC waveform of the commercial power source on the transmission line <b>200</b> is used as a common signal in a plurality of the communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C, each switching circuit <b>140</b> of the communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C is controlled with synchronizing to the AC waveform, in other words, with synchronizing to the synchronizing signals outputted from the AC cycle detector <b>130</b>. Specifically, as shown in <figref idref="DRAWINGS">FIG. 2</figref>, one cycle (60 Hz:16.67 milisec/50 Hz:20 milisec) of the AC waveform sets as control cycle, and the communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C are repeatedly controlled every one control cycle.

0063Specifically, the control in control cycles T<b>2</b>, T<b>3</b>, and T<b>4</b> for acquiring the transmitting right for the data signal frequency band <b>13</b> are performed in control cycles T<b>1</b>, T<b>2</b>, and T<b>3</b> of the control frequency band <b>12</b>, respectively.

0064As shown in <figref idref="DRAWINGS">FIG. 8</figref>, each of control cycles T<b>1</b>, T<b>2</b>, and T<b>3</b> is divided into two periods, specifically, a first half period of the control cycle in the AC voltage waveform (the period from t<b>1</b> to t<b>2</b>) and a second half of the control cycle in the AC voltage waveform (the period from t<b>2</b> to t<b>3</b>). The first half period is used for detecting a carrier. Here, the first half period is set as a period for performing carrier sense multiple access with collision avoidance (CSMA/CA), in other words, a period for carrier detection. The second half period is set as a period for notice of using, in other words, a notice period. That is, with synchronizing to the SS, the CSMA/CA period and the notice period is set.

0065In the CSMA/CA period, each of communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C performs CSMA/CA on the basis of a predetermined back-off-rule. In other words, one of communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C sends the coexist signal transmitted to the control signal frequency band <b>12</b> after the one of the communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C confirms whether the transmission line <b>200</b> is not used for a predetermined consecutive time or more by any of the other communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C. The predetermined consecutive time is a combination of the “waiting time” and the “minimum time.” For example, the minimum time is at least more than one symbol length. If the one symbol length is 100 μsec, the minimum time may be more than 100 μsec, for example, 200 μsec. In this case, the random waiting time is around several tens to hundreds of μsec. Basically, the one of the communication apparatuses <b>100</b>A, <b>100</b>B, and <b>100</b>C that successfully transmits the coexist signal in the CSMA/CA period acquires the transmitting right to exclusively use the data signal frequency band <b>13</b> of the transmission line <b>200</b>.

0066Furthermore, any of several alternative kinds of signals may be used as the coexist signal provided these signals can discriminate carrier existence in the control signal frequency band <b>12</b>. Here, OFDM signals are used as the coexist signal. Multi-tone signals, which have 100 μs symbol length and 56 waves therein, may be used as actual OFDM signals.

0067On the other hand, the notice period in the control period is divided into a plurality of equally spaced parts, for example 8 parts or 16 parts. Each of the equally spaced parts constitutes a notice slot. Therefore, each notice period has 8 or 16 notice slot therein. In <figref idref="DRAWINGS">FIG. 9</figref>, the notice period from t<b>2</b> to t<b>3</b> is divided into 8 notice slots. It is assumed here there are 8 different types of communication methods A, B, C, D, E, F, G, and H, each of which has a different protocol, a different modulation method, and/or a different symbol rate. The notice slot from t<b>21</b> to t<b>22</b> allots to the communication method A, the notice slot from t<b>22</b> to t<b>23</b> allots to the communication method B, the notice slot from t<b>23</b> to t<b>24</b> allots to the communication method C, the notice slot from t<b>24</b> to t<b>25</b> allots to the communication method D, the notice slot from t<b>25</b> to t<b>26</b> allots to the communication method E, the notice slot from t<b>26</b> to t<b>27</b> allots to the communication method F, the notice slot from t<b>27</b> to t<b>28</b> allots to the communication method G, and the notice slot from t<b>28</b> to t<b>29</b> allots to the communication method H. In practice, these different communication methods can be distinguished from manufacturers of the communication apparatuses <b>100</b> or the data communication circuits <b>110</b>.

0068The communication controller <b>120</b> in the communication apparatus <b>100</b> that acquires the transmitting right in the CSMA/CA period in one of the control period transmits the coexist signal to the transmission line <b>200</b> at a timing of its slot allotted to its communication method in the notice period in the same control period. In addition, each of the communication controllers <b>120</b> in each of communication apparatuses <b>100</b> monitors conditions of all slots in the notice period, and confirms whether each of the other communication apparatuses <b>100</b> transmits the coexist signal.

0069In this example shown in <figref idref="DRAWINGS">FIG. 9</figref>, because the communication apparatus <b>100</b>B that uses the communication method B acquires the transmitting right, the communication apparatus <b>100</b>B transmits the coexist signal at the timing of the notice slot from t<b>22</b> to t<b>23</b>. The other communication apparatuses, for example, <b>100</b>A, <b>100</b>C, <b>100</b>D, <b>100</b>E, <b>100</b>F, <b>100</b>G, and <b>100</b>H, which belong to communication methods A, C, D, E, F, G and H, respectively, recognize that the communication apparatus <b>100</b>B that uses the communication method B acquires the transmitting right by monitoring the notice slot from t<b>22</b> to t<b>23</b>.

0070Generally speaking, there is a possibility to collide a plurality of signals with each other if only the CSMA/CA is performed to control a plurality of the communication apparatuses <b>100</b>. In this case, a plurality of the communication apparatuses <b>100</b>, each of which has a different communication method, may acquire the transmitting right in a same CSMA/CA period. The communication apparatuses <b>100</b> that acquire the transmitting right transmit a plurality of the coexist signals therefrom to a plurality of the notice slots in a same notice period. This means that a plurality of signals will come into collision to each other in the next control period. Therefore, the collision should be avoided. Here, all of the communication apparatuses <b>100</b> that acquire the transmitting right monitor all notice slots. When the communication apparatuses <b>100</b> that acquire the transmitting right detect other coexist signal/signals in the same notice period, that is to say, each of the communication apparatuses <b>100</b> that acquire the transmitting right finds the coexist signal/signals in other notice slot/slots, the communication apparatuses <b>100</b> that acquire the transmitting right renounce the transmitting right for the next period to avoid the collision, as explained in detail in the example below. In this case, none of communication apparatuses <b>100</b> transmits data signal in the next data signal frequency band in the next control period.

0071When the other communication apparatuses <b>100</b> that acquire the transmitting right renounce the transmitting right under the same situation mentioned before, a communication apparatus <b>100</b> that acquired the last transmitting right in the last control period will successively acquire the transmitting right. In practice, in a case that the communication apparatus <b>100</b> that acquired the transmitting right in the last control period has data signals in the next control period, the communication apparatus <b>100</b> that acquired the transmitting right in the last control period monitors the notice period of next control period. When the communication apparatus <b>100</b> detects that at least two of other communication apparatuses <b>100</b> send the coexist signal, the communication apparatus <b>100</b> that acquired the transmitting right in the last control period will regard itself as the communication apparatus <b>100</b> that acquires the transmitting right in the next control period, and will occupy the data signal frequency band <b>13</b> in the next control period.

0072An operation above-mentioned will be described, referring to <figref idref="DRAWINGS">FIG. 2</figref>.

0073In <figref idref="DRAWINGS">FIG. 2</figref>, as a result of a CSMA/CA control in a control period T<b>1</b>, a communication apparatus <b>100</b>A that uses a communication method A acquires the transmitting right of a control period T<b>2</b> next to the control period T<b>1</b>. Further, as a result of the CSMA/CA control in the control period T<b>2</b>, a communication apparatus <b>100</b>B that uses a communication method B acquires the transmitting right of a control period T<b>3</b> next to the control period T<b>2</b>.

0074In addition, as a result of the CSMA/CA control in the control period T<b>3</b>, both the communication apparatus <b>100</b>B and a communication apparatus <b>100</b>C that uses a communication method C may acquire the transmitting right of a control period T<b>4</b> next to the control period T<b>3</b>. Then, however, both the communication apparatus <b>100</b>B and the communication apparatus <b>100</b>C recognize by monitoring the other slots in the same notice period that signals from both communication apparatuses <b>100</b>B and <b>100</b>C will collide if each of communication apparatus <b>100</b>A and <b>100</b>C continue to get into communication. Therefore, both communication apparatus <b>100</b>B and <b>100</b>C renounce the transmitting right of a control period T<b>4</b>. After both communication apparatuses <b>100</b>B and <b>100</b>C renounce the transmission right, the communication apparatus B that acquired the latest transmitting right successively acquires the transmitting right of the data signal frequency band <b>13</b> in the control period T<b>4</b>.

0075Furthermore, as a result of CSMA/CA control and monitoring the notice period in a control period, when the communication apparatus <b>100</b> that acquired the latest transmitting right recognizes that no communication apparatuses <b>100</b> acquire the next transmitting right, the communication apparatus <b>100</b> that acquired the latest transmitting right successively acquires the next transmitting right. In other words, the communication controller <b>120</b> of the communication apparatus <b>100</b> that acquired the latest transmitting right monitors whether or not the control signal is produced in each of the slots assigned to each of the communication methods. When the communication controller <b>120</b> of the communication apparatus <b>100</b> that acquired the latest transmitting right finds no produced signals in each slot during monitoring, the communication controller <b>120</b> acquires the next transmitting right. This configuration makes it possible to efficiently use the frequency band because one of communication apparatuses <b>100</b> will always get the transmitting right.

0076As an alternative of the above-mentioned embodiment example, it may also be possible to control that none of the communication apparatuses <b>100</b> transmits data signal in the next data signal frequency band in the next control period.

0077Next, an operation in a condition that there is at least one hidden terminal that is hidden from other terminals will be described. Generally, there is a possibility that one or some of the communication apparatuses <b>100</b> is/are hidden from other communication apparatuses <b>100</b> in an environment that a plurality of communication apparatuses <b>100</b> coexist on the transmission line <b>200</b>. That is to say, in a case that the transmission line <b>200</b> like a power line is used, communication conditions of communication apparatuses <b>100</b>, each of which connects to the transmission line <b>200</b>, dynamically fluctuates in response to line connection status and/or operation status of a variety of electric devices. The fluctuation of the communication conditions may cause the attenuation of signals or a high noise level. Therefore, sometimes at least one of communication apparatuses <b>100</b> connected to the same transmission line <b>200</b> may not be observed by the other communication apparatuses <b>100</b>.

0078In an example shown in <figref idref="DRAWINGS">FIG. 10</figref>, it is supposed that both the home of Mr. X and the home of Mr. Y are in a same housing complex, and communication systems in the X's home and the Y's home, which are different from each other, share the transmission line <b>200</b> (not shown in <figref idref="DRAWINGS">FIG. 10</figref>) in common. In <figref idref="DRAWINGS">FIG. 10</figref>, the left oval shows the X's home, and the right oval shows the Y's home. There are a same kind of three communication apparatuses A<b>1</b>, A<b>2</b>, and A<b>3</b> in the X's home, and there are a same kind of three communication apparatuses B<b>1</b>, B<b>2</b>, and B<b>3</b> in the Y's home. Therefore, all of the communication apparatuses A<b>1</b>, A<b>2</b>, A<b>3</b>, B<b>1</b>, B<b>2</b>, and B<b>3</b> connect to the transmission line <b>200</b> in common.

0079Moreover, in the example shown in <figref idref="DRAWINGS">FIG. 10</figref>, the communication apparatuses A<b>1</b>, A<b>2</b>, and A<b>3</b> can communicate with each other, and the communication apparatuses B<b>1</b>, B<b>2</b>, and B<b>3</b> can communicate with each other. However, because of many reasons such as communication distance between both homes and degradation of frequency characteristic caused by connections of many communication apparatuses to the transmission line <b>200</b>, the communication apparatus A<b>1</b> can be observed by all of the communication apparatuses B<b>1</b>, B<b>2</b>, and B<b>3</b>, and the communication apparatus B<b>1</b> can be observed by all of the communication apparatuses A<b>1</b>, A<b>2</b>, and A<b>3</b>; however, the other combinations among communication apparatuses A<b>1</b>, A<b>2</b>, A<b>3</b>, B<b>1</b>, B<b>2</b>, and B<b>3</b> can not observe each other.

0080Therefore, in the environment shown in <figref idref="DRAWINGS">FIG. 10</figref>, when either the communication apparatus A<b>1</b> or B<b>1</b> acquires the transmitting right, then there is no problem with communication because all of the communication apparatuses can observe the communication apparatuses A<b>1</b> and B<b>1</b>. However, for example, when the communication apparatus B<b>2</b> acquires the transmitting right, the communication apparatuses A<b>2</b> and A<b>3</b> do not recognize that communication apparatus B<b>2</b> connects on the transmission line <b>200</b> because coexistence signal transmitted from the communication apparatus B<b>2</b> in a notice period may not be observed by the communication apparatuses A<b>2</b> and A<b>3</b>, and either the communication apparatus A<b>2</b> or A<b>3</b> also acquires the transmitting right in the next control period in spite of the fact that the communication apparatus B<b>2</b> acquires the transmitting right. Therefore, signals from the communication apparatus B<b>2</b> collide with signals from either the communication apparatuses A<b>2</b> or A<b>3</b>. A control to avoid the collision due to the hidden terminal will be described hereinafter.

0081In the control shown in <figref idref="DRAWINGS">FIG. 11</figref> to avoid the collision due to the hidden terminal, each of the communication controllers <b>120</b> independently monitors frequency of collision of a plurality of signals. When the frequency is high, each of the communication controllers <b>120</b> regards as being at least one hidden terminal on the transmission line <b>200</b>, and performs a specific control to deal with the existence of the hidden terminal(s). Specifically, as an example, each step of the apparatus A<b>1</b> in <figref idref="DRAWINGS">FIG. 11</figref> will be explained hereinafter.

0082In step “S<b>11</b>”, the communication controller <b>120</b> in the communication apparatus A<b>1</b> identifies whether or not the communication apparatus A<b>1</b> acquires the transmitting right of the data signal frequency band in next control period. When the communication apparatus A<b>1</b> identifies to acquire the transmitting right in the S<b>11</b>, then the communication controller <b>120</b> goes through the following step “S<b>12</b> ” to add “1” to a value of a transmitting right counter (the number of acquiring the transmitting right).

0083In the next step “S<b>13</b>”, the communication controller <b>120</b> identifies whether a plurality of the communication apparatuses A<b>1</b>, A<b>2</b>, A<b>3</b>, and B<b>1</b> (B<b>2</b> and B<b>3</b> are hidden from A<b>1</b>) transmit coexistence signals in a plurality of slots in the same notice period, that is, whether a plurality of coexistence signals are outputted in overlapping conditions in the same notice period. When the communication controller <b>120</b> identifies the coexistence signals in a plurality of slots, the communication controller <b>120</b> regards the conditions as occurring the collision, proceeds to the following step “S<b>14</b>”, and adds “1” to a value of a collision counter (the number of occurring the collision).

0084In the next step “S<b>15</b>”, the communication controller <b>120</b> calculates collision frequency (the value of the collision counter/the value of the transmitting right counter), and makes a comparison between the collision frequency and a predetermined threshold (for example, 0.9). The collision frequency tends to be relatively low when no hidden communication apparatus exists on the transmission line <b>200</b>. On the other hand, the frequency of the collision tends to be relatively high when at least one hidden communication apparatus exists on the transmission line <b>200</b>.

0085As a result of the comparison, when the collision frequency is lower than the threshold, the communication controller <b>120</b> regards this condition as indicative of no hidden communication apparatus on the transmission line <b>200</b>, proceeds to a step “S<b>17</b>”, and gets into normal transmission. Meanwhile, as a result of the comparison, when the collision frequency is higher than the threshold, the communication controller <b>120</b> regards this conditions as indicative of at least one hidden communication apparatus on the transmission line <b>200</b>, proceeds to a step “S<b>16</b>”, and stops transmitting signals for a certain amount of time period (the so-called back-off time period). Specifically, the communication controller <b>120</b> stops transmitting a coexistence signal in the CSMA/CA period for a predetermined time period (e.g., 1 to 10 seconds). This control makes it possible to suppress occurrences of the collision in case that at least one hidden communication apparatus is on the transmission line <b>200</b>.

0086Furthermore, in step S<b>17</b>, the back-off time period for which the communication apparatus A<b>1</b> stops transmitting signals is not necessary to be a constant, but may be changeable. For example, a back-off time period of a communication apparatus in which the collision frequency is higher than the threshold may be set longer than those of the other communication apparatuses. That is, if a default back-off time of a communication apparatus, whose collision frequency exceeds the threshold, is the same as a default back-off time of other communication apparatuses, the back-off time of the communication apparatus, whose collision frequency exceeds the threshold, may change to a longer back-off time. For example, if the default back-off time is set at 1 second, the back-off time of the communication apparatus, whose collision frequency exceeds the threshold, may change to 2 seconds, as one example. Furthermore, the communication controller <b>120</b> can return the back-off time period to the original period when predetermined conditions, for example, after a lapse of a predetermined time and at the time when the collision frequency becomes lower than the threshold or other value, is met. For example, the back-off time may change from 2 seconds to 1 second.

0087Another control shown in <figref idref="DRAWINGS">FIG. 12</figref> also provides a measure to deal with the hidden terminal problem. In addition, it is possible to perform the control shown in <figref idref="DRAWINGS">FIG. 11</figref> and another control shown in <figref idref="DRAWINGS">FIG. 12</figref> simultaneously, and it is also possible to perform either the control shown in <figref idref="DRAWINGS">FIG. 11</figref> or another control shown in <figref idref="DRAWINGS">FIG. 12</figref>.

0088The communication apparatuses A<b>1</b>, A<b>2</b>, and A<b>3</b> belong to the same communication system A. When the communication apparatus A<b>1</b> identifies in a slot in the notice period that one of the communication apparatuses A<b>2</b> and A<b>3</b>, in other words, a communication apparatus Ax belonging to the same communication system A, acquires the transmitting right, the communication apparatus A<b>1</b> transmits the coexistence signal to the transmission line <b>200</b> using the same slot as the communication apparatus Ax.

0089In an example shown in <figref idref="DRAWINGS">FIG. 12</figref>, it is assumed that the communication apparatus B<b>3</b> acquires the transmitting right in the same environment shown in <figref idref="DRAWINGS">FIG. 10</figref>. Therefore, the communication apparatus B<b>3</b> transmits the coexistence signal to the transmission line <b>200</b> using B-slots that is the time period from t<b>22</b> to t<b>23</b> assigned to the communication system B in the notice period. As explained previously referring to <figref idref="DRAWINGS">FIG. 10</figref>, the coexistence signal transmitted from the communication apparatus B<b>3</b> is detected by the communication apparatuses B<b>1</b>, B<b>2</b>, and A<b>1</b> because the communication apparatus B<b>3</b> is visible from the communication apparatus B<b>1</b>, B<b>2</b>, and A<b>1</b>.

0090In this case, both the communication apparatuses B<b>1</b> and B<b>2</b> transmit the coexistence signals to the transmission line <b>200</b> when both the communication apparatuses B<b>1</b> and B<b>2</b> detect the coexistence signal transmitted from the communication apparatus B<b>3</b>. Specifically, in response to detecting the coexistence signal from the communication apparatus B<b>3</b> in the first half of the B-slot, both the communication apparatuses B<b>1</b> and B<b>2</b> also transmit the coexistence signals to the transmission line <b>200</b> in the last half of the B-slot. Consequently, the coexistence signal in the last half of the B-slot transmitted from B<b>1</b> is detected by the communication apparatus A<b>2</b>, and A<b>3</b>. Therefore, even in a situation that the communication apparatus B<b>3</b> is hidden from the communication apparatuses A<b>2</b> and A<b>3</b>, the communication apparatuses A<b>2</b> and A<b>3</b> can identify that one communication apparatus belonging to another communication system B acquires the transmitting right. This control makes it possible to prevent the communication apparatuses belonging to either of two communication systems from suffering a collision.

0091Next, another control will be described. For example, when a terminal such as a personal computer accesses to a network like the Internet, it is usual to connect the terminal to a provider using a transmission line like metal line or an optical fiber line. Furthermore, it is also possible to connect between users and the provider using a power line that supplies a commercial power source. Here, a communication method using the power line in a user's home is called an “in-home-system communication method” (hereinafter “in-home-system”). Each of the other communication methods is called an “access-system communication method” (hereinafter “access-system”). Specifically, the access-system includes communication system using a power line connecting between a power pole and each home, or a power line in an office or a factory.

0092In the case of connecting between users and the provider using a power line, both at least one communication apparatus of the in-home-system and at least one communication apparatus of the access-system connect to the common power line connect to the common power line. Therefore, a collision between signals from the access-system and signals from the in-home-system should be also avoided in this case. In addition, regarding the access-system, it is usual that there is only one kind of access-system communication method on a power line if several kinds of communication apparatuses, which are made by different makers, do not connect to the power line. However, it is also possible that, for example, a communication system provided by a communication company and another communication system provided by an electric power company share one power line, that is, there may be a plurality of the access-systems on the same power line.

0093In the above-mentioned cases, basically it is possible to prevent a collision between a plurality kinds of signals by using the afore-mentioned communication apparatus <b>100</b> in the in-home-system. In an example shown in <figref idref="DRAWINGS">FIG. 13</figref>, communication systems A and B show different kinds of in-home-system communication apparatuses <b>100</b>A and <b>100</b>B, respectively. A communication system C shows an access-system communication apparatus <b>100</b>C.

0094In this example, time slots in the notice period mentioned previously, each of which is independent of each other, are assigned to the communication systems A, B, and C. Therefore, each of the communication systems A, B, and C can ensure a frequency band (data signal frequency band <b>13</b>) by performing CSMA/CA control under equal conditions.

0095In this example, by performing a CSMA/CA control in a CSMA/CA period in a control period T<b>1</b>, the communication system A, which is an in-home-system, acquires a transmitting right for the data signal frequency band <b>13</b> in a control period T<b>2</b> next to the control period T<b>1</b>. By performing a CSMA/CA control in a control period T<b>2</b>, the communication system B acquires a transmitting right for the data signal frequency band <b>13</b> in a control period T<b>3</b>. By performing a CSMA/CA control in a control period T<b>3</b>, the communication system C, which is an access-system, acquires a transmitting right for the data signal frequency band <b>13</b> in a control period T<b>4</b>.

0096All of the communication systems including both access-systems and in-home-systems are assigned different identification data, respectively. The control for acquiring the transmitting right is performed based on the identification data. Therefore, this makes it possible to avoid a collision between signals from the in-home-system and signals from the access-system. In addition, it may be possible to assign only one identification data to the entire access-system communication system, or it is also possible to assign different identification data to each access-system communication system. Assigning different identification data to each access-system communication system allows a plurality of access-system communication systems to coexist on a same transmission line. In addition, a coexistence signal comprises the identification data in this example.

0097A modified example will be described with reference to <figref idref="DRAWINGS">FIG. 14</figref>. The longer length of a transmission line that is used for actual communication, the more the high frequency signal component greatly attenuates in the access-system. Therefore, transmission speed in the access-system may not improve even if the frequency band used in the access-system broadens. In that case, for example as shown in <figref idref="DRAWINGS">FIG. 14</figref>, it is preferable to divide the data signal frequency band <b>13</b> into a lower portion (for example, 3 MHz to 10 MHz) and a higher portion (for example, 10 MHz to 30 MHz) and to assign the lower portion to the access-system. Efficiency of the use of frequency thus may be improved.

0098In the example shown in <figref idref="DRAWINGS">FIG. 14</figref>, by performing a CSMA/CA control in a CSMA/CA period in a control period T<b>3</b>, the communication system C acquires a transmitting right for the lower portion in the data signal frequency band <b>13</b> in a control period T<b>4</b> next to the control period T<b>3</b>. The communication system B, which acquired the transmitting right in the last control period T<b>3</b>, again acquires a transmitting right for the higher portion, which is not used by the communication system C, in the data signal frequency band <b>13</b> in a control period T<b>4</b>.

0099In this case, the communication apparatus <b>100</b>B of the communication system B monitors a notice slot allotted to the communication system C in the notice period, in other words, the communication apparatus <b>100</b>B detects the identification data of the communication system C. Thus, the communication apparatus <b>100</b>C recognizes whether the communication system C acquires a transmitting right. If the communication apparatus <b>100</b>B recognizes that the communication apparatus <b>100</b>C acquires the transmitting right to the control period T<b>4</b>, the communication apparatus <b>100</b>B is operable to keep a transmitting right for only the higher portion in the control period T<b>4</b>.

0100Another modified embodiment example will be described with reference to <figref idref="DRAWINGS">FIG. 15</figref>. In the embodiment example shown in <figref idref="DRAWINGS">FIG. 15</figref>, the communication system C does not perform the CSMA/CA control. Instead, the communication system C will acquire a transmitting right for exclusively using the lower portion in the data signal frequency band <b>13</b> as needed. The communication system C will notice in the notice slot in the notice period that the communication system C, which is an access-system, acquired the transmitting right.

0101In this example shown in <figref idref="DRAWINGS">FIG. 15</figref>, it is assumed that no collision occurs between an access-system and an in-home-system. Therefore, although each of the in-home-system A and the access-system C sends a coexistence signal, which represents acquisition of a transmitting right, in a same notice period, this is not a collision, and actually, both the communication system A and the communication system C acquire the transmitting right in the next control period T<b>4</b>.

0102In this case, the communication apparatus <b>100</b>A of the communication system A only uses the higher portion in the data signal frequency band <b>13</b> in the next control period T<b>4</b> if the communication apparatus A detects that the communication system C acquires the transmitting right in the notice period.

0103In addition, it also may be possible that the communication apparatus <b>100</b>C of the communication system C acquires the transmitting right regardless of detecting at least one of control signals sent from other communication apparatuses <b>100</b>A and <b>100</b>B, and sends a control signal related to the acquisition of the transmitting right in the notice period assigned to the communication apparatus <b>100</b>C in advance.

0104Assuming that communication in an apartment is made using a power line, a first user in the apartment may connect the same kinds of communication apparatuses manufactured by a same maker to the power line. However, the more the number of users utilizing power line communication increases, the more different kinds of communication apparatuses may be connected to the power line. In a case that only one kind of communication apparatuses connect to a same power line, the controls mentioned above may not be necessary since no collision between a plurality of signals occurs on the transmission line <b>200</b>.

0105Therefore, another control performed in the communication apparatus <b>100</b> will be described. Each communication apparatus <b>100</b>A, for example, counts how many times each communication apparatus <b>100</b>A or each communication system A, to which each communication apparatus <b>100</b>A belongs, acquires the transmitting right (hereinafter the “acquisition number”). Then the communication apparatus <b>100</b>A compares the acquisition number for a predetermined period with a predetermined threshold (for example, 100 times). Or the communication apparatus <b>100</b>A compares acquisition frequency (the acquisition number/the number of transmitting carrier signals) with a predetermined threshold (for example, 0.99). If the acquisition number or the acquisition frequency is equal to or greater than the threshold, that is, the frequency of acquiring the transmitting right is high, the communication apparatus <b>100</b>A decides that there is no different kind of communication apparatus <b>100</b>B or <b>100</b>C, or communication system B or C in the transmission line <b>200</b>. In this case, the communication apparatus <b>100</b>A fixes the condition of the switching circuit <b>140</b> to be “on” for a certain period (for example, ten minutes or more), and acquires the transmitting right in all control periods. Furthermore, the communication controller <b>120</b> is also operable to stop sending the control signal. Thus, not only aforementioned data signal frequency band <b>13</b> but also the control signal frequency band <b>12</b> can be utilized for data transmission. Therefore, efficiency of the use of frequency may be improved.

0106In addition, as an alternative to the abovementioned control mode, it is possible to perform another control mode described below. The communication apparatus <b>100</b>A counts how many times the other communication apparatuses <b>100</b>B or <b>100</b>C, or the other communication system B or C, acquires the transmitting right (hereinafter the “acquisition number of other system”). Then the communication apparatus <b>100</b>A compares the acquisition number of other system for a predetermined period with a predetermined threshold. If the acquisition number of other system or the acquisition frequency of the other system is equal to or less than the threshold (for example, ten times or 1%), that is, frequency of acquiring the transmitting right by other system is low, the communication apparatus <b>100</b>A decides that there is no different kind of communication apparatus <b>100</b>B or <b>100</b>C or communication system B or C in the transmission line <b>200</b>.

0107As time advances, a different kind of communication apparatus <b>100</b>B or <b>100</b>C may connect to the transmission line <b>200</b>. Even if the communication apparatus <b>100</b>A is under a condition that the communication apparatus <b>100</b>A decided that there was no different kind of communication apparatus <b>100</b>B or <b>100</b>C, or communication system B or C in the transmission line <b>200</b>, and stopped sending the control signal, the communication apparatus <b>100</b>A decides that a different kind of communication apparatus <b>100</b>B or <b>100</b>C is connected to the transmission line <b>200</b> when the acquisition number counted by the communication apparatus <b>100</b>A is not greater than the threshold. Then, the communication apparatus <b>100</b>A changes its control mode to, for example, the control mode shown in <figref idref="DRAWINGS">FIG. 2</figref>.

0108According to these embodiment examples described herein, the communication apparatus controls a communication timing based on a synchronizing signal generated at a specific timing of AC voltage sine-waveform of the commercial power source. Therefore, these examples make it possible to control a variety of timings such as signal transmission, monitoring a timing slot or the like, appropriately. These examples make it possible to prevent signals being a kind of noise from being transmitted to the transmission line by performing such a control that a communication apparatus, which does not acquire a transmitting right, turns off its transmitting function. Therefore, different kinds of signals are prevented from colliding to each other on a power line, and different kinds of communication apparatuses can coexist on a common power line.

0109In addition, although several embodiment examples are described that all control a transmitting right, transmitting a control signal in the notice period and data transmission are performed based on the right timing of the synchronizing signal SS, transmitting a control signal in the notice period and data transmission may be performed starting at a predetermined time after the SS is outputted. The predetermined time is preferable to be less than a half of the AC period; for example, if the frequency of the AC is 50 Hz, then the predetermined time is less than 8.3 milliseconds, for example 3 milliseconds. In addition, the predetermined time is not necessary to be a constant value among all communication apparatuses. For example, each of different kinds of communication apparatuses employing different kinds of specifications may be set a different predetermined time.

0110In some cases, it may not be necessary for the communication apparatus to synchronize with the synchronizing signal SS if the communication apparatus utilizes the synchronizing signal SS, in some way. For example, in a case that the synchronizing signal SS comprises a rectangular wave with one pulse synchronizing with one zero crossing point of AC sine waveform, the communication apparatus may control the transmitting right after a specific time based on the one pulse.

0111Furthermore, if the communication apparatus performs a control of transmitting right via the power line, transmitting a control signal in the notice period and data transmission are not necessary to be performed via the power line. A transmission line for data transmission may be wired or wireless. A variety of cables, such as a LAN cable, a coaxial cable, a telephone line and a speaker line, may be used as the wired transmission line.

0112In addition, although the above-mentioned embodiment examples show that a data signal and a control signal are independent of each other regarding frequency by dividing frequency band into control signal frequency band and data signal frequency band, it is enough if the data signal and the control signal are independent of each other. For example, the control signal band and the data signal band overlap each other, and the data signal and the control signal are transmitted in a time divisional mode.

0113Although preferred embodiment examples have been described and disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutions of these examples are possible, without departing from the scope and spirit thereof.

0114This description is based on Japanese Patent Application No. 2005-000163, filed on Jan. 4, 2005, the entire subject matter of which is expressly incorporated by reference herein.